1. Field of the Invention
The present invention relates to a drive circuit of light-emitting diodes (LEDs).
2. Description of the Related Art
In recent years, a light-emitting device using light-emitting diodes (LEDs) is used as back light source or illumination device of a liquid crystal panel. FIG. 1 is a circuit diagram of formation of a light-emitting device studied by inventors of the present invention, in which the light-emitting device 1003 includes an LED string 1006, a switch power supply 1004 and a current source CS.
The LED string 1006 includes a plurality of LEDs connected in series. The switch power supply 1004 boosts an input voltage Vin and supplies a drive voltage Vout for an end of the LED string 1006.
The current source CS is disposed at a path of the LED string 1006. The current source CS supplies a drive current ILED corresponding to a target luminance for a corresponding LED string 1006. The current source CS includes an output transistor Q1, a current control resistor R4, and an operational amplifier OA. The output transistor Q1 is a PNP bipolar transistor, and is disposed on a path of the drive current ILED. The current control resistor R4 is disposed between a collector of the output transistor Q1 and a ground terminal. An output terminal of the operational amplifier OA is connected to a base of the output transistor Q1, a reverse input terminal of the operational amplifier OA is connected to the collector of the output transistor Q1, and a reference voltage Vref is input to a non-reverse input terminal of the output transistor Q1.
Through the current source CS, feedback is performed in a manner that a collector potential of the output transistor Q1, that is, a voltage drop of the current control resistor R4 is identical with the reference voltage Vref, a current IR4 flowing through the current control resistor R4 is set at a value corresponding to the reference voltage Vref.
The switch power supply 1004 includes an output circuit 1102 and a control integrated circuit (IC) 1100. The output circuit 1102 includes an inductor L1, a switch transistor M1, a rectifier diode D1, and an output capacitor C1. The control IC 1100 adjusts the drive voltage Vout by controlling a duty ratio of ON and OFF of the switch transistor M1. The error amplifier EA amplifies an error between the base voltage Vb of the output transistor Q1 and the reference voltage Vref. The control IC 1100 receives an output signal of the error amplifier EA, and adjusts the output voltage Vout in the manner that the base voltage Vb is identical with the reference voltage Vref.